Ballast employing plurality of temperature sensitive fuses



Nov. 7, 1967 G. L. wou-ERT BALLAST EMPLOYING PLURALITY OF TEMPERATURE SENSITIVE FUSES .Filed July 9,

n n Q uwk NOV- 7,'.1967 l G. l.. woLFET BALLAST EMPLOYING PLRALITY TEMPERATURE SENSITIVE FUSES 2 Sheets-Sheet 2 Filed July 9, 1965 F/ga HMP

Ice 3,351,815

Patented Nov. 7, 1967 United States A*Patent ,l

3,351,815 BALLAST EMPLOYIN G PLURALITY OF TEMPERATURE SENSITIVE FUSES v George L. Wolfert, Danville,-lll., assigner to General Electric Company, a corporation ofkNew Yorkv Filed July 9 196s, ser. No. 470,853 9 Claims. (Cl. 317-40) The invention relates to a ballast for fluorescent lamps, and particularly to ran improved ballast that employs a plurality of temperature sensitive fuses.- i

A temperature sensitive fuse for a fluorescent lamp ballast is described and shown in a patent application entitled, Ballast Apparatus Utilizing Temperature Rey sponsive Fuses, led on Oct. 3,-, 1960, in the name of .Walter C. Mansfield, Jr., Ser.y No. 59,867 (now Patent No. 3,201,646). In that application, a single temperature `sensitive iuse is shown connected inseries with one of' -the input power leads to the lamp ballast. The fuse is internally positionedV at ya location in the ballast that is temperature critical or at a location in the .ballast that indi- Y cates the condition of the ballast. When, for any reason, the temperature at this location Vexceeds a predetermined fuse response temperature, the fuse opens the input lead so as to remove power from thel ballast.l Normally, the fuse `cannot be replaced because of its location. Thus, the opening of the inputlead is permanent so-that the ballast is permanently disabled. Normally, opening of the input lead by the fuse responding to an excessive ternperature and permanent disability of the ballast are preerred to the ballast overheating and damaging the fluorescent lamp, the lamp fixture, and the surroundings.l

But opening of the input lead and permanent disability of the ballast because of a defect in the fuse are wasteful and frustrating. n

Accordingly, an object of the invention is to provide a ballast employing a 'plurality of temperature sensitive fuses that protect the ballastY from overheating.

Another object of the invention is to provide a ballast employing a pluralityy of parallel-connected, temperature sensitive fuses coupled in series with a preselected lead of the ballast to disable the ballast if it becomes overheated, and to maintain theballast in operation as long as oneoffthe parallel-'connected fuses is operative. Another object of the invention is to provide a ballast employing at least two temperature sensitive fusesfthat y ra'reconnected in a parallel circuit, Vthe parallel circuit being'connected in series with one input power lead to the ballastfso that if one fuse operates and .opens' because of a fault in the fuse, the ballast remains operative until the other fuse operates and opens.,y A.

' ABrieliy,`these and other objects are Vachieved in accordance with a preferredfembodimentfof the'invention byl two heat sensitive fuses'connected in 'a parallel electrical circuit. The parallel circuit` is preferably connected in series with one' of" the input'power leads' to theballast.

, 2 panying drawings. The scope of the Vthe claims. Inthe drawings:

FIGURE 1 is a top plan view of a ballast employing two temperature sensitive fuses arranged and connected invention is given in FIGURE 6 is a longitudinal section view of the tem-V perature sensitive fuse shown in FIGURE 4 after the fusey has operated vin response to overheating.

As shown in FIGURES 1V and 2, a ballast employing the two temperature sensitive fuses in accordance with the invention is contained in a suitable metallic case or housing 10. Such a-ballast usually comprises a shell-type f fcore 14 preferably'made of a pluralityof layers of magnetic laminations. The laminations are typically arranged to form an elongated corehaving an inner legV and two side legs. The various ballast windings, which may include a primary winding A, a secondary'winding D, and heater windings G, Hand I, are wound about this inner leg. One orl more capacitors-16, 17 may also be used to provide part of the ballasting action, to improve the Vballast, power factor, and to provide starting. These capacitors y16, 17,1naybe housed in a single can that may be insulated from the case 10. Two temperature sensitive fuses 18,12 are provided, and are preferably Vpositioned at symmetncal and corresponding locations (relativer to the longitudinal axis of the ballast) on the side legs of the core 14 near the windings A, G, H, I as shown in Y FIGURES k1 and 2. Suitable connections are made between the ballast components to provide the electrical circuit-shown schematically in FIGURE 3. After such a ballast is assembled a suitable Iiller 12,V such Ias an :asphalt compound that iselectrically insulatingrand heat The fuses are Ypreferably and `respectively kpositioned on "each side of'the'ballast primary windinginside theballast caseplf one fuseopens because of'a fault in-that one fuse, the other fuse permits theballast to continue operating'. However, iif the ballast actually becomes overheated for any reason, lboth fuses willrespond and open the input power lead. Thus, the desired overheating protection'is provided, butthe disability of the ballast because of a faulty fuse is practically eliminated andthe reliability of the'ballast is` increased. In other embodiments ofl the invention, the parallel circuit maybe connected in series Vwith other ballast leads, or the fuses may be positioned at other locations.

The invention may be better understood Afrom the following description given in connection with' the accomconducting, is used to surround the ballast components. When the llerglz hardens, the ballast components are rmly held inthe case 10. VA suitable metallic cover (not shown), may be placed on top of Vthe ycase. 10 to form a completed assembly.- i' Y v Y In the schematic diagram of FIGURE 3, thevarious electrical elements have theV same reference numerals as their corresponding components showngin-FIGURES l and 2. Thecase 10 is indicated by the dashed lines 10 inA FIGURE 3. The fuses :18,19 are indiatedby rectangles and are coupledin parallel. The' primary winding 'A'and' the Vheater .windingl are coupled in series.' One end of these windings A, I is directly coupled to one side `of an alternating current power source, land the other y Aend of these windings A, I is coupled through the parallelconnected fuses 18,19 to the other side of the power source. One end of` the secondary winding D is coupled to one end of the primary winding A. The primary 1winding A and the secondary winding D are coupled in typical autotransformer relation as indicated by the conventional pclarity dot-s adjacent one end of'each of these windings A, D. In accordance with this dot convention, a given polarity of voltage at the dotted end of one winding ycauses the dotted endfof the other winding to have thesame given polarity of voltage. The other Vend of the secondary winding D is coupled through the power factor capacitor 16 to one side of aA fluorescent lamp 2. The other side of thewlamp 2 is coupledto one side of a fluorescent lamp 1. The other-side of the lamp 1 is cou- Vis reached, and while the pled to the heater winding J. The heaters for the lamps 1 and 2 are heated lby the heater windings G, H, .I as shown. The starting capacitor 17 is coupled across lamp 2. A magnetic shunt may be provided between the primary winding A and the secondary winding D to provide improved ballast operation. When power is applied to the primary winding A, the voltage across the primary winding A and the voltage across the secondary winding D Iare added and applied across the lamp 1 through the starting capacitor 17. After lamp 1 fires, an increased voltage is applied across the lamp 2 so that the lamp 2 is also tired. When the lamp 2 tires, its low voltage drop essentially short circuits the starting capacitor 17 so that current serially flows through the lamps 1 and 2 and through the power factor and .ballasting capacitor 16. As will'be seen in FIGURE 3, the ballast receives power from the alternating current source as long as at least one of the fuses 18, 19 provides an electrical circuit.

Examples of temperature sensitive fuses that can be used for the fuses 18, 19 shown in FIGURES 1, 2, and 3 are described and shown in the patent of Water C. Mansfield, Jr., mentioned previously herein. Another example of such a fuse is shown in FIGURES 4 and 5. FIGURE 4 is a longitudinal section view of one fuse V18 of the two fuses 18, 19 connected in the ballast in accordance with the invention; and FIGURE 5 is a transverse section view of the fuse 18 taken along line 55 in FIGURE 4. As shown in FIGURE 4, the fuse 18 has two leads or wires 20, 21 which may be solid or stranded, and which are connected to other circuits or elements in accordance with the invention. The inner ends of the two wires 20, 21 are joined to a generally cylindrical link 25 which may comprise an alloy of bism-uth and lead. This joining may be accomplished by welding or molding. After the inner ends of the wires 20, 21 are joined to the ends of the cylindrical alloy link 25, the assembly may be encased in a casting 26 which may comprise a wax compound preferably having a melting temperature slightly below the melting temperature of the alloy link 25. If desired, the wires 20, 21 with their alloy link 25 and wax casting 26 may be wound with a suitable insulating tape as shown in FIGURES 1 and 2 to provide protection and strength for the fuse 18 while the ballast is being assembled. After the ballast is connected and assembled with the fuses 18, 19 in the desired location, the ller 12 may be poured around the various parts and the fuses 18, 19 .as well. The fuse 18 shown in FIGURES 4 and 5 is constructed of the alloy and wax compound mentioned, the wax casting 26 being cylindrically shaped and approxi- -mately 1.0 inch long and 0.3 inch in diameter. The fuse is wrapped with tape so that none of the wax casting 26 is visible. This tape protects the fuse during ballast assembly and helps prevent the fuse from prematurely opening when the iiller is poured. Ihe alloy link 25 is .approximately 0;375 inch l-ong and 0.078 inch in diameter. Thus, there is preferably a minimum thickness of the wax casting 26 around the alloy link 25 of at least 0.10 inch. This :fuse 18 is designed to open at temperatures of 118 degrees to 125 degrees centigrade.

Properly constructed fuses will carry the rated electrical current until they are extern-ally overheated to their designed opening temperature. At a Vtemperature below the designed opening temperature of the fuse, the wax casting 26 melts to :provide a molten volume around the alloy link 25. When the designed opening temperature wax casting 26 is still molten, the alloy link 25 melts (and generally tends to collect at one wire) so as'to break the electrical connection between the leads or wires 20, 21. FIGURE 6 shows the fuse 18 after it has been subjected to the designed opening temperature of the fuse 18. It will be seen that there is no electrical or ohmic connection between the wires ance with the invention, one of the fuses 18, 19 opens its electrical path for some reason, such as the ller 12 being too hot, there is still a good chance that the other of the fuses 18, 19 will not open its electrical path. Thus, one of the fuses 18, 19 provides an electrical path for the ballast and permits the ballast to operate under normal conditions. However, after the remaining good fuse is subjected to the design temperature of the fuse, it will open its associated electrical circuit and disable the ballast. But if both fuses 18, 19 are good, as is more likely the case, then the ballast will not be disabled until each of the fuses 18, 19 opens its .respective electrical circuit. The reliability (deiined as the probability that a given device will not fail for some length of time) provided by two parallel-connected fuses may be seen from the following expression:

In this expression, RT is the total reliability of the two parallel-connected fuses, and R1 and R2 are the respective reliabilities of each of the two fuses. If the reliability of each of the two fuses is 0.9, then the total reliability of the two parallel-connected fuses is 0.9-l-0\.9-0.81 or 0.99. Thus, reliability is increased in accordance with the invention. In the case of the two fuses mentioned, the reliability is increased by 10%.

The use of two fuses which must be disabled by an overheated condition to disable the ballast provides an improved ballast. Such fuses are desirable to electrically open and disable the ballast and thereby remove the cause of the overheated condition. This protects the lluorescent lamp iixture, the fluorescent lamp, and their surroundings. However, such fuses are subject to improper operation even though the ambient temperature has not exceeded the predetermined magnitude for the fuse. Therefore, the invention provides two such fuses connected in parallel and preferably located in the vicinity of the primary and heater windings of the ballast. If one fuse opens for an improper reason, the ballast can still function. However, the second fuse will open in response to an overheated condition and disable the ballast. While the invention has been discussed and explained in connection with only one embodiment, persons skilled in the art will appreciate that modifications of the invention may be made. For example, heat sensitive fuses having various dimensions, compositions, materials, and operating characteristics (such as sensitivity to rate of temperature rise) may be used to ll a particular need. The parallel fuses may be connected in either input lead and may be applied to ballast circuits other than the specific circuit shown and discussed. The parallel fuses may also be connected in parts of the ballast circuit other than the input leads. Also, more than two heat sensitive fuses connected in parallel may be provided. Thus, a second pair of parallel connected fuses may be connected in series with the same input lead or the other input lead and placed at desired locations which are likely to become hot. Also, the temperature sensitive fuses may be positioned at locations other than shown in FIGURES 1 and 2. For example, one fuse could be positioned in the vicinity of the primary winding and the other fuse could be positioned in the Vicinity of the secondary winding. In such a case, and also in the case of fuses symmetrically positioned, different designed opening temperatures may be desirable for manufacturing or operating reasons. However, it is generally preferable that similar fuses be positioned in substantially the same corresponding and symmetrical transverse locations relative to the longitudinal axis of the ballast so that both fuses are subjected to approximately the same temperature conditions in the ballast. Therefore, while the invention has been described with reference to a particular embodiment, it is to be understood that modiiications may be made without departing from the spirit of the invention or from the `scope of the claims.

What I claim as new and desire Patent ofthe United States is:

1. In a fluorescent lamp ballast circuit having at least a primary winding and a secondary winding coupled to a magnetic core, said ballast being supplied with electrical power through input leads, the improvement comprising: a plurality of temperature sensitive fuses each having a structure that opens the electric circuit through said fuse in response to the ambient temperature of said fuse exceeding a predetermined magnitude; means connecting said fuses in parallel; means connecting said parallel connected fuses in series with one lead of said ballast circuit; and means positioning said fuses in said ballast case.

2. The improvement of claim 1 wherein said fuses are positioned at spaced locations'relative to each other.

3. The improvement of claim 1 wherein said ballast has a longitudinal axis and wherein said fuses are located substantially in a plane transverse to said longitudinal axis and are located at substantially corresponding and symmetrical points relative to said longitudinal axis.

4. In a uorescent lamp ballast having an elongated shell-type core with an inner leg and two outer legs, having at least a primary winding and a secondary winding wound about said inner leg, and having a housing for said ballast, the improvement comprising: first and second substantially similar temperature sensitive fuses; means electrically connecting said fuses in a parallel circuit; means connecting said parallel circuit in series with said primary winding for connecting said primary winding to an external power source through said fuses; and means respectively positioning said fuses at corresponding locations adjacent said outer legs in the vicinity of said primary winding.

5. In a fluorescent lamp ballast having at least a primary winding and a secondary winding coupled to a magnetic core and housed in a case, said ballast being supplied with electrical power through input leads, the improvement comprising: a plurality of temperature sensitive fuses each having a structure that opens the electric to secure by Letters circuit through said fuse in response to the ambient temperature of said fuse exceeding a predetermined magnitude; means connecting said fuses in parallel; means connecting said parallel connected fuses in series with one of said input leads; and means positioning said fuses in said ballast case.

6. The improvement of claim 5 wherein said fuses are positioned at spaced locations in said ballast case.

7. The improvement of claim 5 wherein said ballast has a longitudinal axis and wherein said fuses are located substantially in a plane transverse to said longitudinal axis and are located at substantially corresponding and symmetrical points relative to said longitudinal axis.

8. In a fluorescent lamp ballast having an elongated shell-type core with an inner leg and two outer legs, having at least a primary winding and a secondary Winding wound about saidfinner leg, and having a housing for said ballast, the improvement comprising: iirst and secondy l temperature'sensitive fuses; means electrically connecting said fuses in a parallel circuit; means connecting said par-` allel circuit in series with one of said windings for connecting said one winding to an electrical circuit through said fuses; and means respectively positioning said fuses at predetermined locations relative to said core.

9. The improvement of claim 8 wherein said fuses have dilferent operating temperatures.

References Cited UNITED STATES PATENTS y2,134,470 10/1938 conrad 317-40 3,201,646 s/1965 Mansfield 317 1s 3,296,518 1/1967 stump6 317-40 FOREIGN PATENTS 665,800 10/1938 Germany.

MILTON O. HIRSHFIELD, Primary Exdmner. R. V. LUPO, Assistant Examiner. 

1. IN A FLUORESCENT LAMP BALLAST CIRCUIT HAVING AT LEAST A PRIMARY WINDING AND A SECOND WINDING COUPLED TO A MAGNETIC CORE, SAID BALLAST BEING SUPPLED WITH ELECTRICAL POWER THROUGH INPUT LEADS, THE IMPROVEMENT COMPRISING: A PLURALITY OF TEMPERATURE SENSITIVE FUSES EACH HAVING A STRUCTURE THAT OPENS THE ELECTRIC CIRCUIT THROUGH SAID FUSE IN RESPONSE TO THE AMBIENT TEMPERATURE OF SAID FUSE EXCEEDING A PREDETERMINED MAGNITUDE; MEANS CONNECTING SAID FUSES IN PARALLEL; MEANS CONNECTING SAID PARALLEL CONNECTED FUSES IN SERIES WITH ONE LEAD OF SAID BALLAST CIRCUIT; AND MEANS POSITIONING SAID FUSES IN SAID BALLAST CASE. 